statx.2: Clarify that the kernel does not reject invalid 'mask' values

And also explain why.

Reported-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/statx.2

@@ -264,11 +264,22 @@ STATX_ALL	[All currently available fields]
 .TE
 .in
 .PP
-.B "Do not"
+Note the kernel does
+.I not
+reject values in
+.I mask
+other than the above.
+Instead, it simply informs the caller which values are supported
+by this kernel and filesystem via the
+.I statx.stx_mask
+field.
+Therefore,
+.I "do not"
 simply set
 .I mask
 to
 .B UINT_MAX
+(all bits set),
 as one or more bits may, in the future, be used to specify an
 extension to the buffer.
 .SS